1. Field of the Invention
The present invention relates generally to a control apparatus for controlling a power transmitting system for an automotive vehicle, including a continuously variable transmission, and more particularly, to a technique for controlling the continuously variable transmission operable in a stepping shift mode wherein the speed ratio is changed in steps based on the currently required output of an engine. In particular, the invention is concerned with a technique for improving the acceleration response of the vehicle to a change in the required output of the engine, and improving the fuel economy of the engine.
2. Discussion of the Prior Art
A control apparatus or method for controlling a continuous variable transmission (hereinafter referred to as "CVT") is known in the art of controlling a power transmitting system of an automotive vehicle. According to one form of such a controller or method, when the currently required output of the engine is smaller than a predetermined upper limit, a target engine speed or a target speed ratio of the CVT is determined so that the actual engine speed is changed only in response to a change in the required output of the engine. When the required engine output is larger than the upper limit, the target engine speed or target CVT speed ratio is determined so that the speed ratio of the transmission is kept at a fixed value. A typical example of such a control apparatus is disclosed in laid-open publication No. 61-136052 of Japanese Patent Application. According to this control apparatus, the CVT maintains the same speed ratio, once an opening angle of a throttle valve of the engine exceeds a predetermined level. This arrangment permits a rapid increase in the vehicle drive force upon rapid or abrupt increase in the throttle opening angle, thereby permitting a relatively high degree of acceleration response, or assuring a powerful acceleration feeling.
However, the control apparatus or method indicated above suffers from a problem due to the use of the same speed ratio of the CVT once the throttle opening angle exceeds the predetermined upper limit. Namely, the engine speed must be raised according to an increase in the vehicle speed, after the speed ratio of the CVT is fixed or after the throttle opening angle reaches the upper limit. This arrangement therefore fails to provide sufficiently high fuel economy of the engine.